1. Field of the Invention
The present invention relates to a laser annealing apparatus, and more particularly to a laser annealing apparatus which is suitable for use in crystallization annealing processes of low-temperature poly-silicon TFT (thin film transistors) and the like.
2. Description of the Related Art
In the crystallization annealing processes of low-temperature poly-silicon TFT, XeCl lasers are widely used as heating light sources. Recently, the fact that green lasers having a high peak power, as represented by the second harmonic of Nd:YAG (yttrium aluminum garnet) lasers having a Q switching action, can be used as heating light sources has attracted great attention because of the ability of such lasers to achieve greater crystal growth at a lower running cost. In annealing processes using green lasers having a high peak power, a system in which a laser beam is formed as a linear beam and directed onto the substrate is most widely used.
FIG. 1 is a schematic diagram showing one example of the construction of a conventional laser annealing apparatus. In a conventional apparatus, a laser beam 206 emitted from a laser oscillator 201 is directly introduced into a linear beam formation optical system 202. The linear beam formation optical system 202 is fixed in place, and the laser beam 206 is propagated using mirrors or the like. The beam intensity is made uniform in the long direction, and the beam is directly focused in the short direction. The laser beam 206 is converted into a linear beam 207 by the linear beam formation optical system 202. The linear beam 207 is directed onto an amorphous silicon substrate 204 which is carried on a stage 208, and the portion of the amorphous silicon substrate 204 that is irradiated by the linear beam 207 is modified to poly-silicon 205. The laser oscillator 201 is carried on a biaxial operating stage 203, and the entire surface of the amorphous silicon substrate 204 is irradiated by the linear beam 207 by scanning the biaxial operating stage 203 in two dimensions.
For example, a laser light linear beam forming method and a laser annealing apparatus using this method are disclosed in Japanese Laid-Open Patent Application No. 2001-156017 as one example of the linear beam formation optical system 202. FIG. 2A is a side view showing the construction of the optical system 300 that forms a linear laser beam as disclosed in Japanese Laid-Open Patent Application No. 2001-156017. FIG. 2B is a plan view of the same, and FIG. 3 is a schematic diagram showing a laser annealing apparatus. Laser light emitted from a laser oscillator 301 is split in the longitudinal direction by a cylindrical lens array 302. The laser light that is split in the longitudinal direction is further split in the lateral direction by a cylindrical lens array 303. Specifically, the laser light that is emitted from the laser oscillator 301 is split into the form of a matrix by the cylindrical lens array 302 and cylindrical lens array 303.
Subsequently, the laser light is temporarily focused by a cylindrical lens 304. In this case, the light passes through a cylindrical lens 305 immediately after the cylindrical lens 304. The light then reflects from a mirror 306, passes through a cylindrical lens 307, passes through a slit 308, and reaches the irradiated surface 309. In this case, the laser light that is projected onto the irradiated surface 309 exhibits a linear irradiating surface. Specifically, the cross-sectional shape of the laser light that passes through the cylindrical lens 307 is linear. The slit 308 is used to adjust the length of the linear laser light in the direction of length.
The homogenization in the short direction of the beam shape of this linearly converted laser light is accomplished by means of the cylindrical lens array 302 and cylindrical lenses 304 and 307. Furthermore, the homogenization in the long direction is accomplished by means of the cylindrical lens array 303 and cylindrical lens 305. The linear beam of the laser light that has thus been made uniform is directed onto a substrate 311 that is carried on a movable stage 310.
However, in cases in which the laser oscillator 201 shown in FIG. 1 has an extremely high output, or in cases in which a plurality of laser oscillators 201 is mounted, the total weight of the optical system, including the linear beam formation optical system 202, reaches several hundred kilograms. Furthermore, if the alignment of the optical system including the laser oscillator 201 is taken into account, the scanning of the linear beam formation optical system 202 lacks stability, is poor in terms of flexibility, and is impractical. Accordingly, systems are now used in which the amorphous silicon substrate 204 is scanned instead of the linear beam formation optical system 202 (for example, see Japanese Laid-Open Patent Application No. 2001-156017).
Methods in which the flexibility of the optical system is improved by propagating a laser beam via optical fibers are also conceivable. However, as the output of the laser oscillator is increased, there is a danger of damage to the optical fibers because of the damage limit of the optical fibers with respect to the laser power density, and the use of small-diameter optical fibers becomes difficult. Accordingly, an extremely high-ratio reducing optical system is required in order to focus the light in the short direction of the beam shape of the linear beam. However, the actual manufacture of such optical systems is extremely difficult. Furthermore, in the case of laser oscillators operated by Q switching, arbitrary control of the laser pulse waveform and spatial intensity distribution is difficult, and conditions that are optimal for working therefore cannot always be obtained.
Recently, in order to solve these problems, a method has been proposed in Japanese Laid-Open Patent Application No. 2002-280324 (pages 4 to 10 and FIG. 1). In this method, a laser oscillating in the near infrared region is caused to perform split propagation by means of a plurality of fibers, the light is amplified by a fiber amplifier and converted into a third harmonic, and the light is then formed into a linear beam and caused to irradiate the substrate.
The construction of the laser apparatus disclosed in Japanese Laid-Open Patent Application No. 2002-280324 is shown in FIG. 4. For example, this laser apparatus 400 is constructed from a near infrared micro-chip laser having an oscillation wavelength of 914 nm, and has a master laser 401 as a laser light source which emits reference laser pulsed light RPref having a wavelength of 914 nm and a pulse width of 0.5 ns; a beam expander 402 comprising two lenses 402a and 402b whose focal points are caused to coincide; a micro-lens array 403 which splits the reference laser pulsed light RPref converted into a large-diameter parallel light ray bundle by the beam expander 402 into N light beams; optical fiber amplifiers 404-1 through 404-N which amplify the split reference laser pulsed light beams DRPref1 through DRPrefN that are propagated, and whose lengths are sequentially set so as to obtain a propagation delay time of 0.5 ns, which is the pulse width of the split reference laser pulsed light beams DRPref1 through DRPrefN; fiber-coupled excitation laser light sources 405-1 through 405-N; excitation light propagating optical fibers 406-1 through 406-N; a third harmonic generating apparatus 407 for receiving the split reference laser pulsed light beams DRPref1 through DRPrefN that have a wavelength of 914 nm and are emitted from the other end surfaces of the optical fiber amplifiers 404-1 through 404-N, and generating third harmonics TRD1 through TRDN having a wavelength of 305 nm and a pulse width of 0.5 ns; and an illuminating optical system 408 in which the laser pulsed light of N third harmonics TRD1 through TRDN obtained by wavelength conversion in the third harmonic generating apparatus 407 is connected and placed in parallel, and pulsed light is emitted whose pulse width is at least N times that of the reference laser pulsed light RPref that has a pulse width of 0.5 ns and is subjected to so-called time multiplexing.
A light wave splitting means is constructed by the beam expander 402 and micro-lens array 403, and a light synthesis means is constructed by the third harmonic generating apparatus 407 and illuminating optical system 408. Furthermore, an excitation light supply means is constructed by the excitation laser light sources 405-1 through 405-N, the excitation light propagating optical fibers 406-1 through 406-N, and optical fiber couplers 414-1 through 414-N.
Furthermore, in Japanese Laid-Open Patent Application No. 2002-280324, it is indicated that the fluence that is required in order to anneal a TFT using an XeCl excimer laser having a wavelength of 308 nm is several hundred J/cm2, and that the pulse width is approximately 20 ns. Moreover, in Japanese Laid-Open Patent Application No. 2002-280324, it is indicated that when a micro-chip laser having a wavelength of 914 nm and a pulse width of 0.5 ns is used as the master laser, it is necessary to amplify the split reference laser pulsed light beams DRPref1 through DRPrefN having a wavelength of 914 nm by means of the optical fiber amplifiers 404-1 through 404-N, and to convert these amplified laser pulsed light beams into third harmonics TRD1 through TRDN having a wavelength of 305 nm and a pulse width of 0.5 ns by means of the third harmonic generating apparatus 407, in order to obtain the necessary fluence described above.
However, in the laser apparatus disclosed in Japanese Laid-Open Patent Application No. 2002-280324, as was described above, it is necessary to amplify the split reference laser pulsed light beams DRPref1 through DRPrefN having a wavelength of 914 nm to a power density of several tens of MW/mm2 or greater by means of the optical fiber amplifiers 404-1 through 404-N. Otherwise sufficient conversion efficiency cannot be obtained at the time of conversion into third harmonics by the third harmonic generating apparatus 407.